fix: add CSSStyleValue to Keyframe and PropertyIndexedKeyframes index signatures

The Web Animations API allows passing CSS Typed Object Model values
(CSSStyleValue and its subtypes like CSSUnitValue, CSSKeywordValue,
CSSTransformValue, etc.) directly to animation keyframes via
element.animate() and KeyframeEffect. However, the current Keyframe
and PropertyIndexedKeyframes interfaces only accept string | number,
causing type errors for valid CSS Typed OM usage.

This adds CSSStyleValue to the index signatures of:
- Keyframe: accepts CSSStyleValue for individual keyframe properties
- PropertyIndexedKeyframes: accepts CSSStyleValue and CSSStyleValue[]

ComputedKeyframe is intentionally left unchanged since getKeyframes()
returns computed string values, not CSSStyleValue objects.

Fixes microsoft/TypeScript#63325

Author: ankitajainparakh

baselines/dom.generated.d.ts

@@ -1386,7 +1386,7 @@ interface Keyframe {
     composite?: CompositeOperationOrAuto;
     easing?: string;
     offset?: number | null;
-    [property: string]: string | number | null | undefined;
+    [property: string]: string | number | CSSStyleValue | null | undefined;
 }
 
 interface KeyframeAnimationOptions extends KeyframeEffectOptions {
@@ -2006,7 +2006,7 @@ interface PropertyIndexedKeyframes {
     composite?: CompositeOperationOrAuto | CompositeOperationOrAuto[];
     easing?: string | string[];
     offset?: number | (number | null)[];
-    [property: string]: string | string[] | number | null | (number | null)[] | undefined;
+    [property: string]: string | string[] | number | CSSStyleValue | CSSStyleValue[] | null | (number | null)[] | undefined;
 }
 
 interface PublicKeyCredentialCreationOptions {

baselines/ts5.5/dom.generated.d.ts

@@ -1383,7 +1383,7 @@ interface Keyframe {
     composite?: CompositeOperationOrAuto;
     easing?: string;
     offset?: number | null;
-    [property: string]: string | number | null | undefined;
+    [property: string]: string | number | CSSStyleValue | null | undefined;
 }
 
 interface KeyframeAnimationOptions extends KeyframeEffectOptions {
@@ -2003,7 +2003,7 @@ interface PropertyIndexedKeyframes {
     composite?: CompositeOperationOrAuto | CompositeOperationOrAuto[];
     easing?: string | string[];
     offset?: number | (number | null)[];
-    [property: string]: string | string[] | number | null | (number | null)[] | undefined;
+    [property: string]: string | string[] | number | CSSStyleValue | CSSStyleValue[] | null | (number | null)[] | undefined;
 }
 
 interface PublicKeyCredentialCreationOptions {

baselines/ts5.6/dom.generated.d.ts

@@ -1383,7 +1383,7 @@ interface Keyframe {
     composite?: CompositeOperationOrAuto;
     easing?: string;
     offset?: number | null;
-    [property: string]: string | number | null | undefined;
+    [property: string]: string | number | CSSStyleValue | null | undefined;
 }
 
 interface KeyframeAnimationOptions extends KeyframeEffectOptions {
@@ -2003,7 +2003,7 @@ interface PropertyIndexedKeyframes {
     composite?: CompositeOperationOrAuto | CompositeOperationOrAuto[];
     easing?: string | string[];
     offset?: number | (number | null)[];
-    [property: string]: string | string[] | number | null | (number | null)[] | undefined;
+    [property: string]: string | string[] | number | CSSStyleValue | CSSStyleValue[] | null | (number | null)[] | undefined;
 }
 
 interface PublicKeyCredentialCreationOptions {

baselines/ts5.9/dom.generated.d.ts

@@ -1383,7 +1383,7 @@ interface Keyframe {
     composite?: CompositeOperationOrAuto;
     easing?: string;
     offset?: number | null;
-    [property: string]: string | number | null | undefined;
+    [property: string]: string | number | CSSStyleValue | null | undefined;
 }
 
 interface KeyframeAnimationOptions extends KeyframeEffectOptions {
@@ -2003,7 +2003,7 @@ interface PropertyIndexedKeyframes {
     composite?: CompositeOperationOrAuto | CompositeOperationOrAuto[];
     easing?: string | string[];
     offset?: number | (number | null)[];
-    [property: string]: string | string[] | number | null | (number | null)[] | undefined;
+    [property: string]: string | string[] | number | CSSStyleValue | CSSStyleValue[] | null | (number | null)[] | undefined;
 }
 
 interface PublicKeyCredentialCreationOptions {

inputfiles/overridingTypes.jsonc

@@ -2923,13 +2923,13 @@
             "BaseKeyframe": {
                 "name": "Keyframe",
                 "overrideIndexSignatures": [
-                    "[property: string]: string | number | null | undefined"
+                    "[property: string]: string | number | CSSStyleValue | null | undefined"
                 ]
             },
             "BasePropertyIndexedKeyframe": {
                 "name": "PropertyIndexedKeyframes",
                 "overrideIndexSignatures": [
-                    "[property: string]: string | string[] | number | null | (number | null)[] | undefined"
+                    "[property: string]: string | string[] | number | CSSStyleValue | CSSStyleValue[] | null | (number | null)[] | undefined"
                 ]
             },
             "BaseComputedKeyframe": {